High performance on-chip inductors and transformers (hereafter sometimes referred as “inductors” or “micro-inductors” for simplicity) are highly desirable in mixed-signal/RF IC (radio frequency integrated circuit) design for increased miniaturization and performance and low power consumption for RF equipment. Past research has resulted in substantial advances in making micro-inductors. Designs ranging from simple stacked planar spiral inductors to complex MEMS (microelectromechanical system) inductors are currently known. However, further improvements over known designs are still desired. Current micro-inductors are very large in size and feature low quality factors (Q-factor) due to magnetic losses. Further, the techniques for manufacturing micro-inductors are not compatible with standard IC fabrication techniques, resulting in greater time and expense to obtain the on-chip inductors.
Complex MEMS techniques have been proposed to make micro-inductors with magnetic cores. For example, micromachined inductors with electroplated magnetic media (NiFe) in lateral bar form for use as a solenoid were reported by Jae Yeong Park and M. G. Allen; Integrated Electroplated Micromachined Magnetic Devices Using Low Temperature Fabrication Processes, IEEE Transactions on Electronics Packaging Manufacturing, Volume: 23 Issue: 1, Jan. 2000, p. 48. Micromachined inductors with electroplated magnetic media (NiFe) in a solenoid thin film spiral format were reported in, D. J. Sadler, Wenjin Zhang, Chong H. Ahn, Hi Jung Kim, and Suk Hee Han; Micromachined Semi-Encapsulated Spiral Inductors For Microelectromechanical Systems (MEMS) Applications; IEEE Transactions on Magnetics, Vol. 33, No. 5, 1997, p. 3319. RF spiral inductors with Co/Fe based sputtered magnetic film and extra planarization (a manufacturing technique used to smooth the surface of an IC wafer), running at 1 GHz and showing a 22% increase in inductance over air-cored devices and a 14% increase in quality factor, Q, have been reported in M. Yamaguchi, M. Baba, K. Suezawa, T. Arai, A. Haga, Y. Shimada, T. Tanabe and K. Itoh, Improved RF Integrated Magnetic Thin-Film Inductors By Means Of Micro Splits And Surface Planarization Techniques, IEEE Transactions on Magnetics, Vol. 36, No. 5, 2000, p. 3495. However, these microelectromechanical-based techniques are not truly compatible with CMOS (complementary metal oxide semiconductor) IC fabrication techniques because many add-on steps outside of traditional wafer fabrication are required in MEMS technology and therefore will add substantial costs to IC fabrication even if successful integration of MEMS and CMOS fabrication techniques could eventually be achieved.
Another obstacle to realizing single-chip RF IC design is that the known IC inductors are all currently too big to fit on a core RF circuit chip. For example, a size of 200 μm×200 μm is a typical footprint size for inductors fabricated in current 0.18 μm CMOS IC fabrication technology to achieve only a few nanoHenries (nH) inductance. By comparison, a typical bonding pad size is only 75 μm×75 μm in 0.18 μm CMOS IC technology. Since an RF IC chip usually needs many inductors, with inductance values ranging from a few nH to a few tens of nH, it is virtually impossible to design single-chip RF ICs using currently available inductor structures due to the large size of the inductors.
Without the development of a compact hi-Q inductor, realizing true single-chip RF ICs and systems-on-a-chip (SoC) will be very difficult and will delay the development of wireless information processing product improvements. It would therefore be desirable to develop new techniques compatible with standard IC fabrication techniques to fabricate compact, high-performance, on-chip inductors. The new compact IC inductors would desirably have a footprint size similar to a typical IC transistor, i.e., a few tens of micrometers (μm) in each dimension.